1. Field of the Invention
This invention relates to cardiac pacemakers, and particularly pacemakers which provide limited bimodal fixed rate pacing for certain patients.
2. Description of the Prior Art
Commonly assigned U.S. Pat. No. 4,932,406, incorporated herein by reference, sets forth the prior art of single and dual chamber pacing and physiologic sensor based rate responsive pacing. Dual chamber DDD physiologic pacemakers following the teachings of U.S. Pat. No. 4,312,355 to Funke may be characterized as possessing a number of different pacing modes which are switched in or out of operation in the presence or absence of atrial and ventricular sensed events. Such DDD pacemakers are constantly updating their functional modes at the end of programmed escape intervals or upon earlier occurring atrial and ventricular events. However, DDD pacemakers do not switch modes in the sense that the expression has been defined in the art.
"Mode switching" connotes a semi-permanent mode change driven by sensed heart activity and/or physiologic sensor derived events occurring in a first relationship wherein the device dictates that it remain in the mode it is operating in until those events assume a second defined relationship. For example, one of the earliest mode switching devices is illustrated by pacemakers exhibiting hysteresis and particularly pacemakers of the type described in U.S. Pat. No. 4,363,325 to Roline, et al, and U.S. Pat. No. 3,999,557 to Citron. In the '325 patent, a pacemaker capable of operating in the atrial synchronous ventricular inhibited (VDD) mode automatically switches to the VVI mode at a preset atrial sensed driven upper rate. The mode is switched back to VDD when a lower atrial sensed driven rate is detected. In the '557 patent, a VVI pacemaker remains "off" until a bout of extreme bradycardia (a heart rate less than 30 bpm) is detected, whereupon the VVI pacemaker switches "on" at a lower rate of 70 bpm, for example. Thereafter, the pacemaker remains in the VVI mode.
Contemporaneously with the introduction of dual chamber pacing, particularly DDD pacing, single chamber and subseguently dual chamber rate responsive pacemakers were developed as described in the aforementioned application. With the introduction and incorporation of physiologic sensors into single and dual chamber pacemakers, a four letter code denoting the modes o operation of pacemakers with and without physiologic sensors and rate adaptive pacing capabilities was published in "The NASPE/BPEG Pacemaker Code" by Berstein, et al, PACE, 10 (4), July-August, 1987, which updated the three-letter code published in the American Journal of Cardiology, 34:487 (1974). The incorporation of physiologic sensors added impetus to the acceptance of the "mode switching" vernacular. However, mode switching is still applied to pacing systems which operate in more than one of the recognized modes in a semi permanent manner as described above.
Current physiologic pacing systems, whether incorporating physiologic sensors of the body's need for oxygen or not, almost always include the components of a DDD pacing system where the preferred mode of operation is to provide atrial and synchronous ventricular stimulation as needed at a rate that varies between a programmed lower pacing rate and upper pacing rate. The advantages of DDD pacing in patients who have a relatively normal SA node function in response to exercise and who do not suffer recurrent episodes of atrial or ventricular tachycardia are well known. However, when such patients suffer episodes of atrial tachycardia and the pacemaker functions at its upper rate limit, cardiac output may decrease and the patient may become distressed. Additionaly, there is a certain danger of inducing ventricular tachycardias in response to sustained operation at the pacing upper rate limit.
Furthermore, in such patients it may be desirable to implant a tachyarrhythmia control device in conjunction with bradycardia pacing capabilities wherein it is necessary to monitor the electrocardiogram to detect the herald signs or the onset of ventricular tachycardia or fibrillation. The operation of the bradycardia pacemaker at the upper rate limit involves the blanking of the sense amplifiers on and off at the upper pacing rate which may make it difficult to detect a developing arrhythmia. Nevertheless, it is desirable to provide some form of backup pacing to treat bradycardia to support patients whose hearts' condition may induce either bradycardia or tachycardia. Conseguently, such tachyarrhythmia control devices have provided for at least single chamber fixed rate bradycardia pacing and DDD pacing has been suggested for inclusion in such systems as set forth, for example, in commonly assigned U.S. Pat. No. 4,548,209 to Wielders, et al. Moreover, it has been recognized that it would be desirable in such tachycardia control systems to include both atrial and ventricular sensing in order to discriminate atrial and ventricular tachycardias and particularly to assist in the discrimination between sinus tachycardias and unstable ventricular tachycardias which may degenerate into life-threatening ventricular fibrillation as described, for example, in my commonly assigned co-pending U.S. Pat. application Ser. No. 07/621,133 filed Nov. 30, 1990.
Finally, in such tachyarrhythmia control devices, antitachycardia pacing therapies are provided which require at least one pacing energy output pulse generator and timing and control circuitry to trigger adaptive or burst overdrive trigger signals.